Michel: to start with I would be delighted to get something that would print just the relevant information for one profile, as Chris noted, the left hand column without all of the other Web page doodles. It would be great if it included a nice printable format for both the Vital data section and the free form section and add page breaks, page numbers, sourced from Wikitree, and a date created. And as far a paper format, it clearly would be best to let the local computer and printer chose the size, but that of course complicates page breaks, page numbers etc.
At the other extreme, it would be a grand slam home run to win the world series to get a piece of software that would automatically grab all of the profiles in a direct line of ascent (and descent) and print them into a pdf book in some sort of an organized format. That sort of output would provide anyone who has taken the time to enter their genealogy into the Wikitree data base and connect it to the master tree the ability to "self-publish," for their own interest, a book of their genealogy, with all of the research, documentation, and biographies present in Wikitree at a particular point in time (hence publication date would also be essential). If it were output in pdf format, the output would retain its hotlinks and could be used and shared in the digital domain or printed from one's computer in paper format. This has some pretty far reaching consequences. First, I would be willing to bet that this could be monetized, I am thinking anyone seriously interested in their genealogy would be willing to pay what it would cost for a bookstore hard copy book to get this PDF file (and a smart entrepreneur would bundle this with some bells and whistles and offer a turnkey printed book probably for several hundred dollars); second an option like has the potential to draw many people with an interest in genealogy into wikitree (the ancestry.com; genealogy.com etc crowd) to enter "the last mile" of their genealogy into Wikitree and to connect with the master tree (hence a feature which could create a push to drive a massive expansion of more recent profiles into the Wikitree database.) and as the reach and quality of content is continually expanding and improving it is likely this could and would be something folks would want to do on a periodic basis.